Sanders responded with his own biting tweet, saying Trump's comment was merely "big talk from the same guy who was too afraid to debate Bernie in California." (Trump had agreed to debate Sanders ahead of the California primary, but backed out of the event saying it was "inappropriate" for him to debate the "second-place finisher" in the Democratic primary.)

Sanders, for his part, attacked Trump in his Clinton endorsement speech, saying he would not fight for any of the progressive policies Sanders lobbied for in the primary.

"Our job now is to see that platform implemented by a Democratic Senate, a Democratic House and a Hillary Clinton president – and I am going to do everything I can to make that happen," Sanders said.

See more from the joint event:

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Hillary Clinton and Bernie Sanders together

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Hillary Clinton and Bernie Sanders together

Democratic U.S. presidential candidate Hillary Clinton and Sen. Bernie Sanders stand together during a campaign rally where Sanders endorsed Clinton in Portsmouth, New Hampshire, U.S., July 12, 2016. REUTERS/Brian Snyder - TPX IMAGES OF THE DAY